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Re: Juvenile characteristics (was) MORE FEATHERS & NEKKID MAMMALS



Betty Cunningham wrote:

>  Forgive me for diverging for a moment here, but isn't it frequently
>found that embryos -during their developement- go through phases which
>display traits which are primitive for that species, such as gills and
>tails in human embryos-which are later re-absorbed into the system?
>  Does this type of throw-back-ness have a technical name?

        I'm not a geneticist, so somebody here correct me if I'm
wrong, but I believe "atavism" is the word you want. "Atavistic"
(primitive) characteristics do show up during embryonic development
(as George pointed out, ontogeny is recapitulating phylogeny) because
they are indeed coded into the genetic material. Generally the
characteristics we now consider primitive (but which in earlier times
might have been called "useful", like heavy jaws for cracking
bones...) show up only briefly, if at all, and are then reabsorbed --
like the gills and tails you mention. Occasionally the genes that are
supposed to suppress these developments malfunction, and we get Jojo,
or babies with tails, or hens with teeth. And, of course, recently
geneticists have begun tweaking those suppressor genes in experiments,
leading to birds with feathers where scutes are supposed to be, and so
on.

>  Is development (and loss) of such characteristics AFTER birth/hatching
>the same sort of thing?  Human babies are frequently born with hair over
>every surface of the body which is lost shortly after birth

Since, as you point out, this is frequent, I'd be rather hard pressed
to call this specifically "primitive" -- it's still with us, even if
it serves no function that we can presently determine. (Remember when
they finally determined that the human vermiform appendix actually DID
serve a function?)

>(unless you have a wierd genetic trait which means you grow up
>looking like Jojo the dog-faced boy).

That would simply be a failure of the suppressor gene.

>  Would it be sense to say in that the Hoatzin and Marsh hen chicks
>(which also have juvenile fingers), that fingers are thus primitive for
>these types and that they have the same sort of delayed absorbtions as
>Jojo does?  Or are these traits found to be some wierd genetically
>triggered thing which was re-evolved specifically for each type?

I thought I remembered reading somewhere that the hoatzin's juvenile
fingers were actually used for climbing around for a while before they
were reabsorbed -- of course, this would be before the wings prepared
themselves for flight. (I could be wrong about this -- somebody please
correct me if I am.) If this is indeed the case, then these atavistic fingers
actually would serve a useful function, and have justified their persistence.

Wayne Anderson